There are two common types of golf balls: thread-wound golf balls and solid golf balls, such as two-piece golf balls. Two-piece golf balls have the advantageous characteristics of having excellent durability and flying distance. On the other hand, a two-piece golf ball is generally harder than that of a thread-wound golf ball, resulting in a loss of feel and control. Feel is the result of many parameters, including hardness, and is difficult to characterize. Improved control, on the other hand, typically arises from a higher spin rate. Because feel and control are highly valued by skilled players, skilled players typically utilize a thread-wound golf ball.
Unfortunately, thread-wound golf balls are relatively difficult to manufacture. Specifically, thread-wound golf balls are manufactured by winding a natural or synthetic rubber thread around a center until the desired diameter is reached. The characteristics of the resulting golf ball are achieved by controlling center size and type, winding tension, thread size, "wound" thickness, cover thickness and cover design. Unfortunately, controlling these characteristics requires relatively sophisticated and expensive winding and tensioning apparatus.
Further, because the elastic thread of wound balls is wound under high tension, there is a risk of the thread snapping during winding, as well as in later manufacturing steps. For example, injection molding generally causes the thread to snap during injection of the molten plastic around it. Thus, the use of thread limits the choice of possible techniques employed to apply the cover around the wound center and the cover is generally compression-molded around the wound.
The concentricity of the wound subassembly is also difficult to control precisely during the manufacturing process. Proper concentricity requires precise adjustment of many parameters, including thread tension, winding speed, and the rotational position of the center. As a result, the manufacture of thread-wound golf balls tends to be relatively delicate and expensive.
Yet another drawback of thread-wound golf balls is the risk of thread snap after manufacture. This risk is exacerbated when the ball utilizes a softer cover, such as balata, which is less resistant to cutting.
Therefore, there is needed a golf ball which has properties of control and feel very similar to a thread-wound golf ball, but which avoids the drawbacks of this construction outlined above.
The invention is an improved solid golf ball, which provides a high spin rate for excellent control and "feel" very similar to a thread-wound ball. Advantageously, the golf ball is particularly adapted to be manufactured in a relatively simple and inexpensive manner. The golf ball also lends itself to multiple methods of producing the cover around the core so that there is greater flexibility in ball construction and manufacture.
One aspect of the invention is a solid golf ball having a core, a cover, an intermediate skin between the core and the cover defining a generally continuous inner annular surface, wherein the inner annular surface of the skin is stretched over the core. Desirably, to prevent breakage, the annular surface is at least 1/4 inch wide. Preferably, the skin has a substantially uniform thickness and covers substantially the entire outer surface of the core so that the golf ball will react in a uniform manner regardless of where on the cover the ball is struck.
The skin may advantageously comprise a single piece, so that the skin may be applied in a single process. Desirably, the inner surface of the skin has a constant diameter so that the skin is stretched an equal amount over the surface of the core. The skin preferably comprises a polymeric material which is stretched so as to have a predetermined amount of initial strain so as to reach a certain level of crystallization of material. This predetermined amount of initial strain is desirably at least 150%, preferably at least 200% and, most preferably at least 400%.
The prestretching of the polymeric material of the skin causes the material to crystallize. This microstructural crystallization abruptly changes its properties of resilience, elastic modulus and viscoelasticity. Specifically, the crystallization tends to increase the elastic properties and cause the material to become stiffer, as opposed to springy. Conversely, the viscoelasticity, which is the capability to absorb energy from shock, decreases so that less absorbed energy is lost and more energy is available to increase the flying distance and spin rate of the ball. That is, more of the energy is restituted.
Alternatively, to facilitate application of the skin over the core, the skin may be comprised of a number of discrete annular bands. To enable the skin to be applied quickly, however, the skin preferably should comprise no more than 8 annular bands.
Advantageously, the golf ball of the present invention is particularly adapted to be constructed with mechanical properties comparable to that of a thread-wound ball, and to be manufactured in a relatively simple and inexpensive manner.
Yet another aspect of the invention is a method for making a golf ball including providing a core, providing a skin defining a substantially continuous inner annular surface having a maximum initial diameter less than the external diameter of the core, applying the skin over the core and molding an outer cover around the skin. Advantageously, the method may further comprise forming the skin by dipping a template in a bath comprising a dispersion of polymeric material.
These and other aspects of the invention will now be described in connection with a preferred embodiment, which is intended to illustrate, rather than limit the invention.